The present invention relates generally to systems and methods for controlling laminar/turbulent flow at a surface and more particularly to system and method for selectively promoting or retarding the laminar/turbulent transition by controlled surface heating.
In a laminar boundary layer on a substantially flat plate, periodic disturbances known as Tollmien-Schlichting waves can develop prior to transition to a turbulent boundary layer. A laminar boundary layer on a surface is considered stable if the disturbances decrease in amplitude as they move downstream along the surface and unstable if the disturbances increase in amplitude. If the disturbances increase in amplitude, a transition to a turbulent boundary layer eventually results.
Experiments by Schubauer and Skramstad (J Aero Sci 14:69-78 (February 1947)) showed that mechanical oscillations of thin ribbons near a surface generate disturbances and influence the transition from laminar to turbulent flow. More recently, Thomas (J Fluid Mech 137:233-250 (1983)) used vibrating ribbons to control or delay boundary layer transition.
Liepmann et al reported (J Fluid Mech 118:187-200,201-204 (1982)) that periodic heating of heater strips could be used to excite laminar instability waves in water and to control, cancel or reduce instability wave amplitudes. By reducing the amplitude of instability waves, transition from laminar to turbulent flow is delayed. Maestrello (AIAA Shear Flow Control Conf. Paper 85-0564 (March 1985)) showed experimentally that surface heating of a boundary layer led to growth of disturbances.
In the prior art teachings, the laminar/turbulent transition is affected by utilizing ribbons spaced away from the surface over which the flow is maintained, and by inducing vibrations in the ribbon mechanically or electromagnetically. Such arrangements suffer from certain shortcomings, particularly in practical application, since mechanical or electromagnetic means are required adjacent the ribbon to provide the required vibration, and mounting the ribbon in spaced relationship to the surface severely disturbs flow along the boundary surface and subjects the ribbon and vibrating means to damaging effects of the flow when not in use.
The present invention solves or reduces in critical importance problems of the prior art as just suggested by controlling (promoting or retarding) the laminar/turbulent transition at a surface using a heated ribbon (or other appropriate strip heater) which is controllably vibrated by pulsed heating to excite vibration in the ribbon, which in turn leads to sinusoidal disturbances in the boundary layer along the surface. Ribbon vibration amplitude and frequency are controlled by selective pulse heating and suitable selection of ribbon attachment geometry. In the practice of the invention, the ribbon is mounted on or in closely spaced relationship to the surface or is flush mounted (recessed) in a groove provided in the surface over which the flow is to be controlled. Inducing vibrations using pulsed heating according to the invention avoids the need for mechanical or electromagnetic vibrating means which necessarily interfere with flow at the boundary surface. Flush mounting is especially desirable as presenting a configuration least susceptible to equipment damage and to interference with boundary layer flow.
The invention has substantial practical utility in the control of boundary layer transition at lifting surfaces on flight vehicles to control stall, to increase lift, to reduce lift loss at high angles of attack, to reduce drag, to counteract pressure oscillations in cavities such as aircraft bays at high subsonic and supersonic Mach numbers, and to improve fuel efficiency. Pulsed heating according to the invention can be selectively applied or removed rapidly. Minimal interference with normal operation of airfoils, lifting surfaces, and the like is realized through use of the invention herein.
It is therefore a principal object of the invention to provide system and method for controlling the laminar/turbulent transition of flow at a surface.
It is a further object of the invention to provide system and method for controllably heating a surface in the control of laminar/turbulent flow at the surface.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.